Spark gaps are generally known types of devices for use as voltage limiters or surge suppressors. Upon a predetermined voltage being applied to the electrodes of the spark gap an arc is initiated therebetween that conducts current and maintains a predetermined voltage level across a protected electrical device. In certain applications of spark gaps very large quantities of current are required to be conducted and this is desirably to be done in a compact and economical device. Also, the device is intended to be capable of repeated operation over a long period of time so that it is important that its characteristics be consistent.
Power gaps of the type to which the present invention is particularly directed have been previously used, for example, to protect series capacitors from overvoltages due to faults or lightning surges on transmission lines. These devices normally consist of two carbon electrodes mounted in an insulating, typically porcelain, enclosure with means provided for the introduction and exhaust of an air (or other gas) blast as the extinguishing medium of the arc. The electrodes used include a top electrode of approximately an umbrella configuration under which is located the bottom electrode in the form of a cylindrical sleeve. The end of the sleeve is spaced a predetermined striking distance from the upper portion of the top electrode at which the arc is initiated. The cylindrical configuration of the bottom electrode permits extinguishing air to be admitted into the enclosure around it and exhausted through the center of the bottom electrode.
Spark gaps as described have been successfully made and used in moderate sizes with capacity up to about 15,000 amperes of fault current. However, a phenomemon is encountered that generally influences power gaps and that is a tendency for the arc after initiation to bow out and transfer to elements other than the carbon electrodes. This tendency may be generally thought of as a seeking of the lowest resistance path between the ultimate conductors connected to the electrodes.
To combat this problem, the present invention uses generally the same configuration as described above with a sleeve of highly conductive material located within the bottom electrode. The arcing tip at the end of this sleeve is disposed near the end of the electrode but is spaced within it a distance, typically approximately equal to the spacing of the first and second electrodes, so the carbon is still subjected to the major impact of arcs. The inner sleeve ensures the arc will travel to the inside of the electrode and be confined there where a desirable conductive material can be used with good lifetime. This has been found to insure that up to considerably higher current levels, such as about 41,000 amperes as compared to structures that previously could carry up to about 15,000 amperes, that the arc will be held at the proper surfaces and that there will be not substantial deterioration of performance.
The arcing tip of the conductive sleeve is preferably a durable conductive material such as Elconite alloy, principally an alloy of silver and tungsten.
Spark gaps in series capacitor protection equipment encounter high current levels because of the large amount of energvy stored in the system that is to be discharged upon occurrence of a fault. For the sake of achieving required performance, a practical constraint has been placed on where the capacitors and their protective gaps are placed in relation to the transmission line. In general, higher fault currents can occur from equipment near the ends of transmission lines than in the middle because less benefit is derived from the impedance of the transmission line itself. Therefore, it has been desirable to work in the middle of the transmission line. There can be occasions, however, where this is unfavorable for the overall system and it would be preferred to be able to work near the end of the line. For example, in one actual system, the magnitude of fault currents near the end of the line can be expected to reach about 40,000 amperes while near the middle of the same line maximum faults of only up to about 7,000 amperes are reached. The present invention provides a power spark gap that gives system design flexibility so that the location of the equipment need not be confined to the middle of the line.